The present invention relates to paper machines and, more particularly to press sections of paper machines.
In particular, the present invention concerns a press section of the closed type in which the paper web leaving the wire section of a paper machine is passed, supported by a first upper fabric, through a first double-felted press nip formed by a pair of hollow-faced rolls. The lower fabric in the double-felted press nip comprises a second fabric which carries the web forwardly after the first nip. The paper web is dewatered in the first double-felted press nip through both faces of the web. A smooth-faced central roll is provided with which at least two single-felt nips are formed. The web is arranged in the press section so as to be detached from the second fabric to adhere to the face of the smooth-faced central roll and to move therewith into the next single-felt press nip, the latter being provided with its own press felt.
The starting point for the present invention was a modification of the well known so-called "Sym-Press" press section ("Sym-Press" being a trademark of applicant's assignee Valmet Oy of Finland) on the basis of several years of operating experience with the "Sym-Press" press section.
The "Sym-Press" press section is disclosed in detail in applicant's assignee's Finnish Announcement Publication No. 50,651. In particular, the "Sym-Press" press section is a compact, so-called fully closed press section in which the paper leaving the wire is passed between two felts through a first nip section constituted by a pair of hollow-faced and/or suction rolls, the paper web being dewatered through both faces of the web. The press section includes a smooth-faced roll which is provided with at least one doctor device. A second nip section is formed against the smooth-surfaced roll by one of the two rolls forming the first nip. The web is dewatered in the second nip through the face of the web which faces the second roll of the first nip. In this conventional press section, at least one additional nip is provided after the second nip, the additional nip being formed between the smooth-faced central roll, whose diameter is greater than the diameters of the other press rolls in the press section, and a hollow-faced roll, a felt passing through the additional nip. The additional nip is situated substantially diametrically opposite the second nip with respect to the central roll.
With respect to the prior art relevant to the present invention, reference is also made to U.S. Pat. No. 4,257,844 as well as to the articles published in the journals Das Papier, Heft No. 1, pages 33-34, 1981, and Norsk Skogindustri, No. 3, page 80, 1974.
In the latter publications, a modification of the "Sym-Press" press section described above is disclosed in which the "Sym-Press" suction roll does not form a nip with the smooth-faced central roll and wherein, either in connection with or prior to the suction roll, the first double-felted press nip of the press section is arranged in which the removal of water takes place in two directions. In lieu of the "Sym-Press" suction roll, a hollow-faced press roll is situated in connection with the smooth-faced central roll to form the second press nip of the press section. The third press nip is formed substantially on the opposite side of the smooth-faced central roll relative to the second nip. In the known press sections described above, it has been necessary to use a suction roll either as a press roll or as a roll within whose area the web supported by the pick-up felt changes its direction upwardly towards the second nip. The use of a suction roll or a corresponding suction device results in considerable drawbacks in the operation of the press section or which are discussed below in greater detail.
The perforations in a suction roll are known to leave a marking on the web which of course is detrimental to the appearance of the paper and which may affect its surface properties. Suction rolls are expensive and require their own drive motors and control systems and are quite noisy. Suction rolls consume large quantities of air since in addition to the air which passes through the web and the felt, air also enters into the suction system which arrives at the suction zone in the holes or perforations in the suction roll mantle during each revolution of the roll. Moreover, the water seals of the suction box of the suction roll can cause difficulties during operation.
In particular, as is known, a suction roll comprises a rotating cylindrical perforated mantle and a stationary axial suction box situated within the cylindrical mantle and which is sealed to the inner face thereof. The.suction width of the suction box is, for example, 100 to 500 mm and the suction box extends from one end of the mantle to the other. The suction box is connected to a suction system so that an airflow is produced through the holes in the area of the mantle which faces the suction box at any particular time during rotation of the roll. It can be seen from the foregoing that suction rolls are relatively expensive components of paper machines. For example, the drilling of the perforations in the mantle is a difficult and expensive job and the perforations inherently reduce the strength of the mantle thereby making it necessary to use special metal alloys in the construction of the rolls and relatively high thicknesses for the mantle which results in high material costs.
As noted above, the quantity of air situated within the holes in the suction roll passing into the suction zone to enter into the suction system is surprisingly high in the case of modern, high-speed paper machines. With higher speeds of the paper machine, greater amounts of "hole-air" enter the suction system. The proportional amount of "hole-air" is further increased by the fact that when the speed of the machine is increased, the rolls must be made even stronger, such as by increasing the thickness of the mantle. Of course, increasing the thickness of the mantle proportionally increases the quantity of hole-air carried into the suction system. As also noted above, an additional drawback inherent in suction rolls is the high noise created during their operation.
Especially in the case of suction rolls used as press rolls, it is often necessary to arrange for a variable crown for the roll. However, this is usually not possible since the mantle of a suction roll is perforated and/or since the interior of the roll is occupied to such an extent by the suction box that no devices for varying the crown can be fitted within the interior of the mantle. It of course would be desirable to eliminate this drawback.
As noted above, a suction roll and, in particular, a press suction roll, is a highly expensive component of a press section of a paper machine.